Recent development of advanced information society is remarkable. The multimedia, in which pieces of information in a variety of forms are integrated, rapidly comes into widespread use. A magnetic disk apparatus, which is installed to a computer or the like, is known as one of the multimedia. At present, development is advanced for the magnetic disk apparatus aiming at the miniaturization while improving the recording density. Concurrently therewith, development is also advanced quickly in order to realize a low price of the apparatus.
In order to realize the high density on the magnetic disk, for example, it is demanded that (1) the distance between the disk and the magnetic head is narrowed, (2) the coercivity of the magnetic recording medium is increased, (3) the signal-processing method is executed at a high speed, and (4) the thermal fluctuation of the magnetic recording medium is reduced.
In order to realize the high density magnetic recording on the magnetic recording medium, it is necessary to increase the coercivity of the magnetic film. A material based on the Co—Cr—Pt(—Ta) system has been widely used for the magnetic film of the magnetic recording medium. This material is a crystalline material in which crystal grains of Co of about 20 nm are deposited. In order to realize, for example, an a real recording density exceeding 40 Gbits/inch2 on the magnetic recording medium in which such a material is used for the magnetic film, it is necessary to further decrease the size of the unit (magnetic cluster) in which the inversion of magnetization occurs during recording or erasure, and it is necessary to decrease the grain size distribution so that the structure and the organization of the magnetic film are precisely controlled. When the control is made as described above, the noise, which is generated from the medium during the reproduction, can be reduced. However, any dispersion arises in the crystal grain size. Especially, when grains having small sizes exist in the magnetic film, then the thermal demagnetization and the thermal fluctuation take place, and the magnetic domain, which is formed in the magnetic film, fails to stably exist in some cases. Especially, when the magnetic domain is made fine and minute as the recording density is increased, remarkable influences are exerted by the thermal demagnetization and the thermal fluctuation. In order to reduce the noise generated from the medium by making the crystal grains to be fine and minute, the thermal fluctuation is suddenly increased. Especially, when the crystal grain diameter is not more than 10 nm to 8 nm, the thermal fluctuation has appeared conspicuously. For this reason, in view of the reduction of the thermal demagnetization and the thermal fluctuation, it becomes an important technique to control the crystal grain size distribution. As a method for realizing the above, for example, U.S. Pat. No. 4,652,499 discloses a method in which a seed film is provided between a substrate and a magnetic film.
However, the magnetic disk, which uses a ferromagnetic film as a magnetic film, has involved a certain limit of control of the magnetic grain diameter and the distribution thereof in the magnetic film by using the method in which the seed film is provided as described above. For example, when the super high density recording exceeding 40 Gbits/inch2 is performed, the grain diameter distribution has been broad with large-sized grains and minute grains existing in a mixed manner, for example, even when the material for the seed film, the film formation condition, and the structure of the seed film are adjusted. When information is recorded (when magnetization is inverted), the minute grains are affected by influences of leak magnetic field from surrounding magnetic grains. On the other hand, the large-sized grains interact with surrounding magnetic grains. Further, some magnetic grains, which have grain diameters larger than the average of those of the magnetic grains, cause the increase in noise when recording/reproduction is performed. Other magnetic grains, which have grain diameters smaller than the average, sometimes increase the thermal fluctuation when recording/reproduction is performed. For this reason, it has been difficult to reliably record information. As a result of the presence of magnetic grains having a variety of sizes in a mixed manner in the mass of magnetic grains, the boundary line between an area in which inversion of magnetization occurs and an area in which inversion of magnetization does not occur exhibits a rough zigzag pattern as a whole. Such a phenomenon also causes the increase in noise.
In order to perform the high density recording, it is also important that the magnetic layer is thermally stable. The value represented by (Ku·V)/(k·T) can be used as an index for the thermal stability of the magnetic layer. In the expression, Ku represents the magnetic anisotropy energy, V represents the volume of activation, k represents the Boltzmann's constant, and T represents the temperature. As the value is increased, the magnetic layer is thermally stable. Therefore, in order to enhance the thermal stability of the magnetic layer, it is necessary to increase the volume of activation V and the magnetic anisotropy energy Ku. This fact also holds in the same manner as described above for the magnetic film for perpendicular magnetic recording based on the Co—Cr system.
In the case of the crystalline material based on the Co system, when the magnetic grains are made fine and minute, it is anticipated that the magnetization possessed by the magnetic-crystal grains may be changed, because of transfer into the mesoscopic region. As a result, it is further difficult to secure the heat resistance of the magnetic film.
In order to satisfy the requirement as described above, it has been investigated that an amorphous alloy as a ferrimagnetic substance, which is composed of rare earth element and iron family element, is used for a magnetic film for recording information. For example, it has been reported in 23rd Annual Meeting of Magnetic Society of Japan (8aB11, 1999) that a rare earth-iron family alloy as an amorphous material is hopeful as a magnetic material which is excellent in thermal stability and which is preferred for high density recording. In “InterMag 2000 HA-04”, a medium, in which an amorphous alloy based on the rare earth-iron family is used for a recording film, is disclosed as a magnetic recording medium which is resistant to the thermal fluctuation. Although such an amorphous alloy is excellent in thermal stability, the magnetic wall is liable to move. Therefore, when a magnetic field is applied during the recording of information to record the information, it has been difficult to stably form the minute magnetic domain in a magnetic layer. For this reason, it has been necessary that the magnetic wall position (corresponding to the information bit position) is determined highly accurately so that the position of the magnetic domain may be correctly established during the recording of information. This inconvenience results from the fact that the rare earth-transition metal alloy is a magnetic material of the magnetic wall movement type.
The present invention has been made taking the foregoing situations into consideration. A first object of the present invention is to provide a magnetic recording medium which has a large volume of activation of a magnetic film, which has high thermal stability, and which is excellent in reproduction performance, and a magnetic recording apparatus provided with the same.
A second object of the present invention is to provide a low noise magnetic recording medium in which the shape of the magnetic domain hardly takes a form of zigzag pattern in a magnetization transition area and the zigzag pattern is not reflected, and a magnetic recording apparatus provided with the same.
A third object of the present invention is to provide a magnetic recording medium which has large magnetic anisotropy, which is excellent in stability of recorded information, and which makes it possible to reliably form the minute magnetic domain, and a magnetic recording apparatus provided with the same.
A fourth object of the present invention is to provide a magnetic recording medium which has a simplified stacked (laminated) structure and which is suitable for mass production, and a magnetic recording apparatus provided with the same.
A fifth object of the present invention is to provide a magnetic recording medium which makes it possible to highly accurately determine the position of the magnetic wall (i.e., the position of the magnetic domain) formed in an amorphous magnetic film during recording of information, and a magnetic recording apparatus provided with the same.
A sixth object of the present invention is to provide a magnetic recording medium which is preferred for the super high density recording exceeding 40 Gbits/inch2 (6.20 Gbits/cm2), and a magnetic recording apparatus provided with the same.